The present invention relates to methods for preventing or minimizing discoloration of elastomer rubbers and the products formed thereby. More particularly, the present invention relates to the addition of polar functional groups to ethylene propylene elastomer rubbers to prevent discoloration, even after weathering and exposure to ozone.
It is common in the motor vehicle industry to fashion extruded and molded elastomeric rubber parts for use in a variety of applications, including glass run channels, gaskets, hoses, wear strips and weather seals. Various elastomeric rubbers, such as ethylene-xcex1-olefin elastomers, including ethylene-propylene-diene terpolymer (EPDM) rubbers and ethylene-propylene (EP) rubber, and styrene-butadiene copolymer rubber (SBR), have been used to form these rubber parts. These materials are favored by manufacturers because they are relatively inexpensive compared to other elastomers and generally exhibit the desired flexibility necessary for a number of applications as well as acceptable weatherability properties.
Ethylene-xcex1-olefin elastomers, particularly ethylene-propylene-diene (EPDM) rubbers, are excellent all-purpose rubbers, especially suitable for a wide variety of automotive applications. As used herein, rubber is defined to mean a polymeric material that is capable of recovering from large deformations quickly and forcibly and is essentially insoluble in solvents. EPDM rubber is a rubber comprising ethylene and propylene repeating units with a smaller amount of diene units to effect crosslinking of the polymer chains.
In many formulations of EPDM rubber used in the automotive industry, various additives are incorporated into the base rubber. These additives can include carbon black and various processing oils. Carbon black is used as a filler and reinforcing agent and to improve ozone and UV-light resistance. Processing oils are used to reduce the viscosity of the rubber, which provides advantages in processing. The EPDM rubbers are typically cured using a sulfur or peroxide cure system. Sulfur cure systems are generally preferred since they are effective, inexpensive and require no special processing techniques.
While EPDM parts provide good performance in many automotive applications, they often suffer from surface discolorations. These surface discolorations include sulfur blooming, in which the sulfur or other curatives migrate to the surface of the molded or extruded part imparting a crystalline whitish film on the part surface, and iridescence (or xe2x80x9cbronzingxe2x80x9d), which is the phenomenon of xe2x80x9coil on waterxe2x80x9d color formation in carbon black filled rubber. Iridescence often manifests itself as a yellowish, greenish or bluish sheen on the part surface. As used herein, iridescence and bronzing are used interchangeably and both refer to this non-crystalline color formation on a rubber surface resulting from curatives or other additives migrating to the rubber surface. These surface discolorations grow more pronounced when the part is subjected to weathering, oxygen and ozone exposure.
The discoloration of EPDM parts by sulfur bloom or iridescence does not affect the physical properties of the part, but it is aesthetically displeasing to many in the automotive industry. Thus, various studies have been conducted in an attempt to eliminate or reduce this discoloration. The origin and mechanisms of iridescence and blooming are not fully understood. However, several factors have been found to affect the amount of discoloration exhibited by a sample. These factors include the identity and amount of fillers, the use of different accelerators and the curing conditions. Based on the identification of these factors, various approaches have been taken in an effort to reduce discoloration. The use of specific grades of sulfur as well as specific vulcanization accelerator compositions have shown modest success. However, a noticeable discoloration is still apparent in state of the art sulfur-vulcanized, oil and carbon black loaded EPDM rubbers after weathering and ozone exposure.
Thus a need exists for an EPDM rubber that shows no discoloration even after subjected to weathering and oxygen and ozone exposure.
The present invention is directed to a method of producing ethylene-xcex1-olefin rubber compositions that do not exhibit iridescence, blooming or other discoloration phenomenon often associated with such rubbers, and the compositions formed thereby. The principle of the present invention is based on formulating the rubber, including a specific curing system, in a manner that has been found to reduce or eliminate the appearance of iridescence and other sheen phenomenon that has been observed in EPDM and other ethylene-xcex1-olefin elastomers.
In a first aspect, the effect of varying the base rubber, accelerators, and cure conditions was investigated. It was determined that the use of a lower ethylene content EPDM will decrease the amount of iridescence in the final product. Preferably, an EPDM base compound containing less than 58% ethylene by weight is used. Likewise, a lower accelerator content will help to minimize iridescence. Preferably, less than 4 parts per hundred resin (phr) of total accelerator is incorporated into the rubbers of the present invention. In addition, a high degree of cure will also decrease iridescence. Preferably, cure time is greater than 5 minutes at 204xc2x0 C.
In a second aspect, it was discovered that the addition of polar functional groups to the rubber prevented discoloration in a state of the art optimized EPDM rubber. It is believed that the groups increased the polarity of the compound, which increased the solubility of compounds responsible for iridescence and thus decreased their migration to the rubber surface. Preferably, the polar functional groups are grafted onto a non-polar compound, such as EPDM or ethylene-propylene rubber, which is then introduced into the base rubber composition. The non-polar compound is miscible with the base rubber, and allows the grafted polar functional groups to disperse within the base rubber. Alternatively, the polar functional groups may simply be monomer units in a polymer, such as a styrene-butadiene copolymer. It was found that the addition of maleic anhydride grafted polyethylene to the rubber composition nearly completely eliminated any discoloration on the surface of the cured rubber, even after ozone aging for 24 hours.